Avalanches pose an extremely grave danger for skiers, climbers, military personnel and others in mountain regions during winter and spring. Death or serious injury can result when a person is buried in snow unless rescue is quickly effected. Rescue is difficult because a person buried in an avalanche cannot be seen, may be moved by the avalanche away from the last visible position and cannot easily be located. Suffocation, hypothermia and cardiac arrest, coupled with shock, become more likely the longer the victim is buried. The amount of time spent in locating a victim is an important factor in the probability that the victim will not survive or will be severely injured.
In an attempt to address these problems, electronic locating systems are increasingly being used, for example, by the ski industry and the military. With one type of system presently in use, each member of a group exposed to avalanche danger is supplied with a continuously energized, battery powered radio transceiver unit operating continuously in a transmitting mode. With these known systems, continuous transmission is relied upon to assure that transmission occurs when a victim is buried. If part of the group is buried in an avalanche, the remaining members switch their units to a receiving mode and attempt to locate buried individuals by variations in transmitted signal strength.
This type of system has serious drawbacks. Power output is limited because of the battery drain during continuous operation. Since the units must be easily portable and battery weight is limited, the use of high power alarm signals is impractical. The long wavelength transmission frequencies of these conventional units provide a non directional signal. In order to find a buried victim, it is necessary to move about, preferably in a grid pattern, while attempting to detect increases and decreases in signal strength. This can be difficult and time consuming, particularly in the unfavorable terrain common to avalanche sites. The difficulty is compounded by the fact that survivors are often confused or in shock. In addition, if multiple victims are buried, the region of maximum signal strength may not coincide with the location of a victim. Power consumption and the necessity for continuous transmission prevent the use of an audible alarm that would be useful when a survivor is in relatively close proximity to a victim.
Another type of system currently in use relies on reflected microwave signal detection rather than radio transmission. Each member of a group wears a reflector such as a simple dipole antenna coupled to a diode. A powerful microwave transceiver unit is used to scan the area of an avalanche with a beam of energy. Reflection or backscatter of energy at the second harmonic frequency indicates the presence of a buried victim. One problem is that a transceiver unit of this type is very expensive and, due to the energy required, is too bulky and heavy to be easily transported by a skier or climber. It is not practical for each member of the group to carry a transceiver unit, and the benefit of each survivor being able to search immediately and electronically for victims is lost. Following an avalanche, it may be necessary to transport the radar unit from a distance, and the lost time can result in death or injury of the victims.